Recently, information-processing systems using an electrophotographic method are making a remarkable progress. In particular, laser printers and digital copiers which record data with light by changing the data into digital signals make remarkable improvements in their printing qualities and reliabilities. Further, technologies used in these printers and copiers are applied to laser printers and digital copiers capable of printing full-color images with high-speed printing technologies. Because of these reasons, photoreceptors are required both to produce high-quality images and to have high durability.
Photoreceptors using organic photosensitive materials are widely used for these laser printers and digital copiers due to their cost, productivity and non-polluting properties. The organic photoreceptors are typically classified to a single-layered type and a functionally-separated type. The first practical organic photoreceptor, i.e., PVK-TNF charge transfer complex photoreceptor was the former single-layered type.
In 1968, Mr. Hayashi and Mr. Regensburger independently invented PVK/a-Se multi-layered photoreceptor. In 1977, Mr. Melz, and in 1978, Mr. Schlosser disclosed multi-layered photoreceptors whose photosensitive layers are all formed from organic materials, i.e., an organic-pigment dispersed layer and an organic low-molecular-weight material dispersed polymer layer. These are called as functionally-separated photoreceptors because of having a charge generation layer (CGL) generating a charge by absorbing light and a charge transport layer (CTL) transporting the charge and neutralizing the charge on the surface of the photoreceptor.
However, the photosensitive layer of the organic photoreceptor is easily abraded due to repeated use, and therefore potential and photosensitivity of the photoreceptor are likely to deteriorate, resulting in background fouling due to a scratch on the surface thereof and deterioration of density and quality of the resultant images. Therefore, abrasion resistance of the organic photoreceptor has been an important subject. Further, recently, in accordance with speeding up of printing and downsizing of image forming apparatus, the photoreceptor has to have a smaller diameter, and durability thereof has become a more important subject.
As a method of improving the abrasion resistance of the photoreceptor, methods of imparting lubricity to the photosensitive layer, hardening the photosensitive layer, including a filler therein and using a polymeric charge transport material (CTM) instead of a low-molecular-weight CTM are widely known. However, another problem occurs when these methods are used to prevent the abrasion of the photoreceptor. Namely, an oxidized gas such as ozone and NOx arising due to use conditions or environment, adheres to the surface of the photosensitive layer and decreases the surface resistance thereof, resulting in a problem such as blurring of the resultant images. So far, such a problem has been avoided to some extent because the material causing the blurred images are gradually scraped off in accordance with the abrasion of the photosensitive layer.
However, in order to comply with the above-mentioned recent demand for higher sensitivity and durability of the photoreceptor, a new technology has to be imparted thereto. In order to decrease an influence of the material causing the blurred images, there is a method of equipping the photoreceptor with a heater, which is a large drawback for downsizing the apparatus and decreasing power consumption. In addition, a method of including an additive such as an antioxidant in the photosensitive layer is effective, but since a simple additive does not have photoconductivity, and a large amount thereof in the photosensitive layer causes problems such as deterioration of the sensitivity and increase of residual potential of the resultant photoreceptor.
As mentioned above, the electrophotographic photoreceptor having less abrasion by being imparted with abrasion resistance or a process design around thereof inevitably produces blurred and low-resolution images, and it is difficult to have both of high durability and high quality of the resultant images. This is because high surface resistance of the photosensitive layer is preferable to prevent the blurred images and low surface resistance thereof is preferable to prevent the increase of residual potential.
Most of the electrophotographic photoreceptors in the market are functionally-separated photoreceptors each including an electroconductive substrate, a CGL and a CTL layered thereon, and a CTM included in the CTL is a positive hole transport material. These are mostly used in negatively-charging electrophotographic processes.
A reliable charging method in the electrophotographic processes is a corona discharge, and most of copiers and printers use the corona discharge. As widely known, a negative-polarity corona discharge is more unstable than a positive-polarity corona discharge, and therefore a scorotron charging method is used, resulting in one of cost increase elements. The negative-polarity corona discharge generates more ozone causing chemical damages, and when used for a long time, the ozone oxidizes a binder resin and a CTM, and ionic compounds produced in charging such as nitrogen-oxide ions, sulfur oxide ions and ammonium ions accumulate on the surface of a photoreceptor, resulting in deterioration of image quality. Therefore, ozone filters are used in negatively-charging copiers and printers to prevent the ozone from discharging out in many cases, resulting in cost increase. Further, a large amount of ozone causes environmental pollution.
In order to solve these problems, positively-charged electrophotographic photoreceptors are being developed. The positively-charged electrophotographic photoreceptors generate less ozone and NOx ions, and further produce stable images with less environmental variation with two-component developers widely used at present.
However, the positively-charged single-layered or reverse-layered photoreceptor has a drawback of varying in its properties due to an environmental gas such as exhaust gases from blue heaters and cars because of including a charge generation material (CGM) vulnerable to oxidizing materials such as ozone and NOx ions at the surface.
In contrast, the negatively-charged electrophotographic photoreceptor is preferably used rather than the positively-charged electrophotographic photoreceptor in a high-speed copy process. This is because an organic material having high charge transportability even in the high-speed copy process is at present almost limited to a positive hole transport material, and a normally-layered electrophotographic photoreceptor having a CTL including a positive hole transport material at the surface is limited to be negatively charged in principle.
As mentioned above, a positively and negatively chargeable electrophotographic photoreceptor can further expand its applications, and is advantageous for cost reduction due to model reduction and higher speed.
Japanese Patent No. 2732697 discloses a positively and negatively chargeable electrophotographic photoreceptor. However, a diphenoquinone derivative as an electron transport material used therein has slightly low charge transportability, and the photoreceptor does not have sufficient sensitivity for higher speed and smaller copiers and printers. Further, the photoreceptor has a drawback of producing blurred images after repeatedly used.
Japanese published unexamined application No. 2000-231204 discloses an aromatic compound having a dialkylamino group as a deoxidizer. This compound is effectively included in a photoreceptor to produce quality image even after repeatedly used. However, the compound having low charge transportability is difficult to comply with higher sensitivity and speed, and therefore a content thereof has a limit.
Further, a stilbene compound having a dialkylamino group disclosed in Japanese published unexamined application No. 60-196768 and Japanese Patent No. 2884353 has an effect on the blurred images due to the oxidizing gas on page 37 of Konica Technical Report Vol. 13 written by Itami, etc. and published in 2000.
However, since the compound has a substituted dialkylamino group having a strong mesomeric effect (+M effect) at a resonance portion in its triarylamine structure, which is a charge transport site, total ionization potential is extremely small. Therefore, the compound has a critical defect of being quite difficult to practically use because charge retainability of a photosensitive layer in which the compound is used alone as a CTM largely deteriorates from the beginning or after repeated use. In addition, even when the above-mentioned stilbene compound is used together with other CTMs as it is in the present invention, the compound has a considerably smaller ionization potential than the other CTMs and becomes a trap site against a charge transport, and therefore, the resultant photoreceptor has quite a low sensitivity and a large residual potential.
Japanese published unexamined application No. 2004-258253 discloses a photoreceptor including a stilbene compound and a specific diamine compound having improved environmental resistance to repeated use and oxidizing gases without deterioration of sensitivity.
However, this is not sufficient for a high-speed printing photoreceptor having a smaller diameter.
Because of these reasons, a need exists for an electrophotographic photoreceptor having high durability against repeated use for a long time, preventing deterioration of image density and blurred images and stably producing quality images.